Phase-shifting mask and semiconductor device

ABSTRACT

Disclosed is a phase-shifting mask having a pattern comprising a plurality of substantially transparent regions and a plurality of substantially opaque regions wherein the mask pattern phase-shifts at least a portion of incident radiation and wherein the phases are substantially equally spaced, thereby increasing resolution of a given lithographic system. Further disclosed is a semiconductor device fabricated utilizing the phase-shifting mask.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.09/488,662, filed Jan. 20, 2000, now U.S. Pat. No. 6,638,663 B1.

TECHNICAL FIELD OF THE INVENTION

This invention relates to lithographic masks, such as that used forfabricating semiconductor devices.

BACKGROUND OF THE INVENTION

Lithography is utilized in semiconductor device manufacturing to patternfeatures on semiconductor workpiece layers for integrated circuitfabrication.

FIG. 1 shows a lithographic fabrication system 100 for defining featuresin a workpiece 120, in accordance with prior art. Typically, workpiece120 comprises a semiconductor substrate, together with one or morelayers of substances (not shown) such as silicon dioxide and a resistlayer 101, affixed to a surface of workpiece 120.

Typically, radiation of wavelength λ is emitted by an optical source106, such as a mercury lamp or a laser. The radiation propagates throughan optical collimating lens or lens system 104, a patterned lithographicmask 103 having a pattern of opaque and transparent features, and anoptical projection lens or lens system 102. The radiation transmittedthrough mask 103 is imaged by lens 102 onto resist layer 101, therebyexposing a patterned area corresponding to the mask pattern. If resistlayer 101 is positive, exposed areas will be subject to removal afterdevelopment and if it is negative, exposed areas will remain intact.Thus, the pattern of mask 103 is transferred to (“printed on”) resistlayer 101. “Mask” as used herein means “mask” and/or “reticle”.

As known in the prior art, the indicated distances L₁ and L₂ satisfy, incases of a simple lens 102, 1/L₁+1/L₂=1/F, where F is the focal lengthof lens 102. A pattern produced by mask 103 on resist layer 101 will besubstantially in focus if resist layer 101 is a distance L₂ fromprojection lens 102. This conclusion is based on a geometrical opticsanalysis which assumes light travels in straight lines. However, whenthe feature size is comparable to λ/NA, where λ is the illuminationwavelength, and NA is the numerical aperture of the projection lens, aphysical optics analysis should be considered which includes the wavenature of light. Under this analysis diffraction effects are likely tobe produced, decreasing the image resolution even at distance L₂,thereby reducing resolution of component features. For semiconductordevices it is desirable to maximize the number of circuit components perunit area by minimizing component size. As component size decreases,diffraction effects become more significant, thereby limiting reductionin component size. Decreased sharpness of mask images caused bydiffraction effects may reduce product yield and increase device failurerate.

Diffraction effects may be severe for conventional or binary masks. FIG.2A depicts a cross-sectional view of a prior art binary mask 10. Binarymask 10 typically comprises a glass or quartz layer 12 with a patternedchromium layer 40 affixed thereto. The patterned chromium layercomprises a plurality of substantially transparent areas 14, 15 and 16and a plurality of attenuating areas 18, 19, 20 and 21. Electromagneticradiation propagating through areas 14, 15 and 16 have electric fieldsassociated therewith. Amplitudes of the electric fields at the masklevel are represented with respect to a cross-section of the mask inFIG. 2B, wherein steps 36, 37 and 38 correspond to electric fields fromradiation propagating through apertures 14, 15 and 16, respectively.Because of the wave-nature of the radiation it spreads as it propagates.Therefore, even though the electric fields are separated from oneanother at mask level they may interfere with one another a distanceaway from the mask, such as at a workpiece surface. This is shown inFIG. 2C. Due to the diffraction effect, it is clear that the electricfield at the workpiece surface spreads wider relative to that at themask level. The smaller the feature sizes, as represented by transparentareas 14, 15, and 16, the wider the spread.

Solid lines 22 and 24 in FIG. 2C represent electric fields fromapertures 14 and 16, respectively, and broken line 26 represents anelectric field from aperture 15. The amplitudes of the electric fieldsfrom adjacent openings (14 and 15, for example) overlap in cross-hatchedregions 30 and 32. As shown in FIG. 2D, this interference orconstructive addition of electric field amplitudes results in anelectric field 34 which has a higher intensity at the workpiece surfacein regions 30 and 32, relative to the surrounding areas than at masklevel. Therefore, there is less contrast in the light intensitydistribution at the workpiece surface than at mask level, therebyreducing the resolution capability of the tool.

Undesirable diffraction effects become more significant with smalldimension pattern features. “Small dimension” as used herein means smallsize and small spacing between transparent regions relative to λ/NA,where λ is the wavelength of the optical source and NA is the numericalaperture of the projection system.

It is known in the art to improve the system resolution by employingphase-shifting masks. The mask imparts a phase-shift to the incidentradiation, typically by π radians. Phase-shifting masks generallycomprise transparent areas having an optical intensity transmissioncoefficient T, near 1.0 at the incident radiation wavelength λ,attenuating areas or partially transparent areas having T at λ in therange of about 0.05 to about 0.15, and, optionally, opaque areas, havingT less than or equal to about 0.01.

FIG. 3A depicts a cross-sectional view of a prior art πradian-phase-shifting mask 300. Mask 300 is substantially similar tobinary mask 10 but includes a phase-shifter layer 310 over transparentregions 14 and 16. Phase-shifter layer 310 reverses the direction of theelectric field vectors at apertures 14 and 16 relative to aperture 15 asshown in FIG. 3B at 320, 322 and 330. The π radian phase-shift iscreated by employing a phase-shifter layer 310 with a thickness of d=λ/2(n−1) where λ is the wavelength of the optical source and n is therefractive index of layer 310 at λ. The phase-shifter layer modifies theoptical distance traveled by incident radiation, thereby producing aphase-shift. As is shown in FIG. 3C, by peaks 340, 345 and 350, theoverlapping regions of adjacent electric fields have oppositeamplitudes, and therefore, a destructive interference occurs. Thecancellation of the electric field at those locations improves thecontrast of the intensity field as shown in FIG. 3D. FIG. 3E depicts avector diagram of the electric field at a workpiece level produced byradiation propagating through a π radian-phase-shifting mask. Vector 380represents an electric field from unshifted radiation such as passesthrough aperture 15. Vector 390 corresponds to phase-shifted radiationsuch as that which propagates through aperture 14 and phase-shifter 310.The amplitude of vector 390 equals the negative of the amplitude ofvector 380, thereby canceling it out upon interference.

Phase-shifting masks producing π radian shifts are an improvement overbinary masks. However, they do not fully resolve all resolutionproblems, for example a phase conflict may arise for featureconfigurations in which a phase transition is generally unavoidable.Whenever a phase transition occurs a dark line will result.

Electric field interference has been addressed by using a mask having aπ/2 radian shift and a 3/2 π radian shift. Liebmann et al, “AlternatingPhase Shifted Mask for Logic Gate Levels, Design and MaskManufacturing”, SPIE vol. 3679 p. 27 (1999). It is also known in the artto use π:2/3 π:1/3 π:0 radian shifting masks.

It is therefore desirable to reduce phase conflict thereby substantiallyeliminating undesirable lines, and thus facilitating feature sizereduction and improving product yield and reliability.

SUMMARY OF THE INVENTION

The invention relates to a phase-shifting mask having substantiallyequally spaced phases thereby substantially eliminating zeroth order andreducing first order diffraction frequencies. One embodiment of theinvention relates to a three-phase-shifting mask having a patterncomposed of substantially transparent regions and substantially opaqueregions wherein the mask pattern phase-shifts incident radiation by 0,2/3 π and 4/3 π radians. The invention further relates to asemiconductor device fabricated utilizing the phase-shifting mask. Insuch applications the invention facilitates reduction in component sizeand improved device reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art lithographic system useful in the practice ofthe invention.

FIGS. 2A–2D depict a prior art binary mask.

FIGS. 3A–3E depict a prior art π radian phase-shifting mask.

FIGS. 4A–4E depict a three-phase-shifting mask of the invention.

FIG. 5 depicts Fourier spectra of a three-phase-shift mask, a π radianphase-shift mask and a binary mask.

FIG. 6 depicts a mask pattern.

FIG. 7 depicts a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

It will be appreciated that the following description is intended torefer to specific embodiments of the invention selected for illustrationand is not intended to define or limit the invention, other than in theappended claims.

The invention comprises a phase-shifting mask having substantiallyequally spaced phases such that the zeroth order diffraction frequencyis substantially canceled and the first order diffraction frequency isreduced as compared to nonphase-shifting masks or masks having unequallyspaced phases. Any number of equally spaced phases may providesubstantially similar pattern transferring results and are within thespirit and scope of the invention. However, the phase-shifting maskpreferably has three equally spaced phases to simplify manufacturing.Phase-shifting masks having phase shifts of 1/3π radian multiples can befabricated by layering readily available 1/3π radian phase-shiftingcomponents.

FIG. 4A depicts a cross-sectional view of a three-phase-shifting mask400. Three-phase-shifting mask 400 has a plurality of substantiallytransparent areas 402, 404 and 406 and a plurality of substantiallyopaque areas 410, 412, 414 and 416. Extending across apertures 402 and404 are phase-shifters, 420 and 422, respectively. Phase-shifter 420produces a 2/3 π radian shift and phase-shifter 422 produces a 4/3 πradian shift. FIG. 4B shows the amplitudes of electric fields at masklevel wherein the field areas from apertures 402 and 404 are representedby negative steps 430 and 432, respectively, and the field area fromaperture 406 is represented by positive step 434. Since the electricfields are vectors by nature, FIG. 4B should be understood as a snapshotof the fields at a specific moment that will continually change withtime. FIG. 4C represents electric fields 440, 442 and 444 at a workpiecefrom apertures 402, 404 and 406, respectively. Unlike the binary mask,the electric fields at the overlap region are added destructively.Therefore, where images on the workpiece surface from apertures 402, 404and 406 meet, the intensity is substantially zero as shown in FIG. 4D at450 and 452.

This phenomenon is further depicted in FIG. 4E. FIG. 4E depicts electricfield vectors corresponding to an electric field at workpiece level formask 400. It should be noted that the amplitudes of the electric fieldsare the projection of the vectors shown in the figure to the verticalaxis. Vector 460 corresponds to an electric field produced by aperture406 through which unshifted radiation is propagated. Vector 462represents an electric field at workpiece level produced by radiationpropagating through aperture 402 which is phase-shifted 2/3 π radians byphase-shifter 420. Vector 464 defines an electric field at workpiecelevel of radiation propagating through aperture 404 and 4/3 π radianphase-shifter 422. The amplitude of vectors 462 and 464 aresubstantially equal when vector 460 is at its maximum amplitude as shownin FIG. 4E. The vector array rotates clockwise with time at a frequencydetermined by the frequency of the incident radiation. As vector 460rotates, its amplitude will decrease. As the amplitude of vector 460decreases, the amplitude of vector 462 will become more negative and theamplitude of vector 464 will become less negative. However, the sum ofthe amplitudes of vectors 460, 462 and 464 will remain generally equalto zero, thereby substantially eliminating light intensity at thelocation where the electric fields overlap. For masks having any numberof substantially equally spaced phases, corresponding electric fieldvectors will generally sum to zero.

Advantageously, the frequency component of three-phase-shift mask 400 islower than binary mask or π radian phase-shifting mask 300. This makesit possible for radiation to pass through the limited numerical apertureof the projection lens, and therefore achieve higher resolution with agiven system. This phenomenon will also be present for masks with othernumbers of equally spaced phases.

FIG. 5 depicts Fourier spectra of a binary mask, a π radianphase-shifting mask and an equally spaced three-phase-shifting mask. Thezeroth order diffraction frequency is substantially eliminated and thefirst order diffraction frequency is reduced with thethree-phase-shifting mask as compared to the binary and π radianphase-shifting masks. Binary mask spectrum 510 indicates a first orderdiffraction frequency centered at C. π radian phase-shifting maskspectrum 520 has a first order diffraction frequency centered at Bindicating a lower frequency. Advantageously, three-phase-shifting maskspectrum 530 shows a center of its first order diffraction frequency tobe at A indicating an even lower frequency than that of the π radianphase-shifting mask. Lower diffraction frequency corresponds to improvedresolution. Therefore, resolution with a three-phase-shifting mask willbe better than that with a binary or π radian phase-shifting mask,thereby facilitating formation of smaller features. Other equally spacedphase-shifting masks should produce results similar to those obtainedfrom the three-phase-shifting mask.

FIG. 6 depicts one example of a mask pattern in which a phase conflictis likely to occur with a π radian shift. FIG. 6 shows three opaque maskfeatures 610, 620 and 630 surrounded by transparent areas 640, 650, 660,670 and 680. If a π phase-shifting mask is employed to shift radiationpropagated through transparent areas 640 and 660 by it radians, andradiation propagated through areas 650 and 670 are left unshifted or atzero, the electric field interference produced by diffraction ofradiation propagating through transparent areas 640, 650 and 660 will beminimized. However, transparent area 680 has portions adjacent totransparent areas 640, 650 and 660 so that a phase transition isunavoidable between either 680 and 650 or between 680 and 640/660. Wherethe phase transition occurs, an undesirable dark line will usually beproduced. This phenomenon is referred to as “phase conflict”.

Advantageously, substantially equally spaced phase-shifting masks reducephase conflict. For example, for the mask pattern depicted in FIG. 6, byintroducing a third phase and having the phases equally spaced, features650, 660 and 680 can have different phases from one another, therebysubstantially eliminating phase conflict. Furthermore, transparent area640 can have the same phase as transparent area 660 without producing aphase conflict. Because interference of the electric fields from thethree features is substantially eliminated, unwanted dark lines willgenerally be eliminated.

The preferred mask thickness will depend on its application and on themask material. For example, in a photolithographic process used in thefabrication of semiconductor devices the mask thickness is preferably inthe range of about 0.22 cm to about 0.64 cm. It will be understood bythose skilled in the art that any mask thickness will be suitable thatallows the transmission of radiation sufficient to transfer the maskpattern to the workpiece and which has the structural integritynecessary to withstand the process in which it is used.

The preferred mask material will also depend on the application forwhich the mask is used. For example, masks typically comprise glass orquartz when used in photolithographic processes in the manufacture ofsemiconductor devices. Any material sufficient to withstand theparticular lithographic process for which it is used and through whichsufficient radiation may be transmitted to transfer the mask pattern tothe workpiece may be utilized. Additional examples of mask materialsinclude, but are not limited to, silicon dioxide fluorides, alkalinemetals fluorides and alkaline earth fluorides. Calcium fluorides andmagnesium fluorides are particularly well suited as mask materials.

In a lithographic process radiation is propagated through the mask andfocused with a lens onto a workpiece coated with resist. If a negativeresist is used, exposed areas will remain intact. If a positive resistis employed, exposed areas will be removed. By this process, the patternof the mask will be transferred to the workpiece. Areas in which resisthas been removed may then undergo additional processes, for exampleetching and plating, to form features on the workpiece in a desiredpattern.

The invention further includes a semiconductor device which, when formedusing a substantially equally spaced phase-shifting mask, should havebetter feature definition than that which is formed using a prior artmask, primarily due to improved resolution. FIG. 7 depicts a schematicof a semiconductor device 200 that may be formed using a substantiallyequally spaced phase-shifting mask. Those skilled in the art willunderstand that it shows a simplified drawing of semiconductor device200 for illustrative purposes only. An actual device may have layers ofvarying thicknesses and may contain other components. Semiconductorsubstrate 202 is covered by a first dielectric layer 204. Above firstdielectric layer 204 is a first metal layer 206. Vias or interconnects208, 210, 212 and 214 penetrate layer 204 and conductively connect firstmetal layer 206 to semiconductor substrate 202. First metal layer 206 iscovered by second dielectric layer 216 which contains vias 218, 220 and222 to connect first metal layer 206 to a second metal layer 224. Thislayering sequence may be repeated as necessary as shown in part bylayers 226 and 228, and interconnects 232, 234 and 236. A toppassivation layer 230 may be applied to protect device 200 from adverseelectrical, chemical or other conditions, and to provide electricalstability.

Semiconductor substrate 202 may comprise silicon, for example. Commondielectrics include, but are not limited to, silicon oxides, such asboron phosphorous doped silicate glass (BPSG), those originating fromtetraethylorthosilicate (TEOS) and silicon dioxide (SiO₂). Common metalsinclude, for example, aluminum, copper and tungsten. In addition, toimprove adherence between metal and dielectric layers, thin layers maybe introduced between them. Titanium is commonly used for this purpose.Electronic circuitry is defined in the layers by a lithographictechnique.

In the lithographic process used to form the circuitry in device 200 aresist is deposited over a dielectric layer. The resist is exposed bytransmitting radiation through the substantially equally spacedphase-shifting mask onto the dielectric layer surface, thereby definingdesired circuitry and substantially eliminating phase conflict. The formof radiation used is dependent on the type of resist and otherfabrication parameters. Any form of radiation that may expose the resistwithout adverse effects to the workpiece may be used. Common examplesinclude, ultraviolet radiation, electron beam radiation and x-rays. If apositive resist is used, the exposed areas will be removed revealing thedielectric layer below. The dielectric layer may then be removed, forexample by etching. Any technique that will remove the exposeddielectric layer while leaving the resist covered portions intact mayalso be used. Negative resists may be used wherein the exposed resistareas are left intact after exposure and the nonexposed areas areremoved. For negative resist processes a mask is used that defines thespaces between circuit components rather than the circuitry itselfLithographic processes using the substantially equally spaced phaseshifting mask may also be employed to form other device features, forexample interconnects in the dielectric layers.

The phase-shifting mask described herein is not limited in use tosemiconductor device fabrication and may, within the spirit and scope ofthe invention, be used for any lithographic process in which it wouldfacilitate transfer of a pattern to a workpiece.

1. A a semiconductor device comprising an integrated circuit formed on asubstrate the semiconductor device being free of dark line transfererrors from a phase-shifting mask used to pattern the substrate withphases that are substantially equally spaced such that theircorresponding electric field vectors sum to zero and the unwanted darklines are essentially eliminated from the transferred pattern appearingon the surface of the substrate.